To provide an account of genetic and social determinants of aggressive behavior, this application proposes a longitudinal, multigenerational study of health-related behavioral factors in social development, using an animal model. Human aggression exacts a heavy toll in the health of individuals. The costs of risk- taking in psychopathological and antisocial interactions are incurred by victims of violence, by victims of their own risk- taking, and by societal loss of cohesion and social and medical remediation. Generally, the perpetrators of violent acts are adolescent and young adult males. The research proposed here will clarify the developmental and genetic factors that produce sex differences in aggressive behavior. The pattern of similarities and differences in aggressive expression among females and males is of general interest because of its implications for understanding genetic, hormonal, physiological, and social role processes. One of the more important findings to emerge from previous work was that when gender-appropriate assessment conditions were employed, strong evidence was obtained for cross-sex similarities in the heritability of aggressive behavior in mice. Line differences in females directly paralleled line differences in males, even though only the aggressive behavior of males was selected. Virtually no empirical evidence is available on the development course of aggressive expression in females. This information is essential in order to establish precise correspondences (and differences) between the sexes, and to clarify the mechanisms of aggressive behavior transmission and control in each sex. The proposed work will employ new methods of investigation to study the 15th and 16th generations, focused on: 1) the interactions of line, sex, and developmental timing in the ontogeny of aggressive behavior; 2) the organization and the extent of individual consistency in two forms of female aggressiveness: intruder-elicited aggression, and maternal aggression; 3) the stability of aggressive behavior in individuals over the life span; 4) the progressive effects, over generations, of selective bredding for male aggressiveness.